Method and installation for production of noble gases and oxygen by means of cryrogenic air distillation

ABSTRACT

A method and an apparatus for producing oxygen and noble gases by cryogenic distillation. A first stream of cooled and purified air is sent to a medium pressure column where it is separated. A first nitrogen enriched stream is withdrawn from the medium pressure column, and part of this stream is sent to a low pressure column. An intermediate stream is withdrawn from an intermediate level of the medium pressure column. This intermediate stream is then sent to the low pressure column. A stream, which is enriched in oxygen relative to the intermediate stream, is withdrawn from the bottom of the medium pressure column and sent to an auxiliary column. The auxiliary column also receives at liquid stream of nitrogen which is used for reflux. A second nitrogen rich stream is withdrawn from the top of the low pressure column. A second oxygen rich liquid stream, which is suitable for use as a product, is withdrawn from the low pressure column. Finally, a final oxygen enriched stream which is also enriched with krypton and xenon, is withdrawn from the auxiliary column.

The present invention relates to a method and a plant for producingoxygen and rare gases by air distillation.

A weak krypton/xenon mixture is conventionally produced from a purge atthe main vaporizer of a double air separation column (see“Tieftemperaturtechnik” by Hausen and Linde, 1985 edition, pp. 337-340and “Separation of Gases” by Isalski, 1989 edition, pp. 96-98). Theoxygen produced is then withdrawn from the low-pressure column a fewstages above the vaporizer. If the oxygen is withdrawn in gaseous form,this arrangement allows a substantial fraction of the krypton present inthe air and all of the xenon to be recovered.

However, in the case of a unit producing oxygen by what are called“pumped” methods, about 30% of the krypton and of the xenon present inthe air are “lost” in the liquid oxygen withdrawn from the low-pressurecolumn.

DE-A-2603505 discloses an air separation unit in which a fluidcontaining krypton and xenon is produced in a purification column fedwith two streams of rich liquid coming from the medium-pressure column,the reboiling in the purification column being provided by a vaporizerfed with the overhead gas from an argon column.

One object of the present invention is to propose systems for increasingthe krypton and xenon yield of units producing gaseous oxygen by pumpingand vaporization of liquid oxygen (or more generally those withsubstantial withdrawal of liquid oxygen from the bottom of thelow-pressure column) and, preferably, also producing argon.

Another object of the present invention is again to have a principalvaporizer with a high oxygen content and massively purged, and thus togreatly limit the concentration of hydrocarbons/impurities (theadvantage of a pumped “oxytonne”), which is not the case with theconventional scheme producing a weak mixture of krypton and xenon.

One subject of the invention is a method for producing oxygen and raregases by distillation in a column system comprising at least onemedium-pressure column, one low-pressure column and one auxiliarycolumn, in which method:

-   -   i) at least one stream of cooled and purified air is sent to the        medium-pressure column where it is separated;    -   ii) at least a first nitrogen-enriched stream is withdrawn from        the medium-pressure column and at least one portion of this        stream is sent directly or indirectly to the low-pressure        column;    -   iii) an intermediate stream is withdrawn from an intermediate        level of the medium-pressure column;    -   iv) a stream, enriched with oxygen relative to the intermediate        stream, is withdrawn from the bottom of the medium-pressure        column and sent to the bottom of the auxiliary column;    -   v) a nitrogen-rich stream is withdrawn from the top of the        low-pressure column;    -   vi) an oxygen-rich liquid stream is withdrawn from the        low-pressure column as product, optionally after a vaporization        step in order to form a gaseous product; and    -   vii) an oxygen-enriched stream, which is also enriched with        krypton and with xenon relative to the second oxygen-enriched        stream, is withdrawn from the auxiliary column, characterized in        that the intermediate stream is sent to the low-pressure column        and a liquid stream containing at least 78 mol % nitrogen is        sent as reflux to the auxiliary column.

Preferably, the liquid stream sent as reflux to the auxiliary column isliquefied air and/or liquid enriched with nitrogen relative to aliquefied air stream sent to the medium-pressure column. According tooptional aspects:

-   -   the bottom of the auxiliary column is heated by an overhead gas        from an argon column;    -   the liquefied air and/or the liquid enriched with nitrogen        relative to the air is produced by heat exchange with the        oxygen-rich liquid stream coming from the bottom of the        low-pressure column, optionally after a pressurization step;    -   the nitrogen-enriched liquid contains at least 80 mol %        nitrogen;    -   the liquefied air does not come from the medium-pressure column,        the liquid stream sent to the top of the auxiliary column is        richer in nitrogen than the intermediate stream; at least 10% of        the oxygen produced is withdrawn in liquid form from the        low-pressure column.

Another subject of the invention is a plant for producing oxygen andrare gases by distillation in a column system comprising at least onemedium-pressure column, one low-pressure column and one auxiliarycolumn, which plant comprises:

-   -   i) means for sending at least one stream of cooled and purified        air to the medium-pressure column where it is separated;    -   ii) means for withdrawing at least a first nitrogen-enriched        stream from the medium-pressure column and means for sending at        least one portion of this stream directly or indirectly to the        low-pressure column;    -   iii) means for withdrawing a nitrogen-rich stream from the top        of the low-pressure column;    -   iv) means for withdrawing an intermediate stream from an        intermediate level of the medium-pressure column;    -   v) means for sending a stream, richer in oxygen than the        intermediate stream, from the bottom of medium-pressure column        into the bottom of the auxiliary column;    -   vi) means for sending a liquid stream as reflux to the auxiliary        column;    -   vii) means for withdrawing an oxygen-rich liquid stream from the        bottom of the low-pressure column as product, optionally after a        vaporization step in order to form a gaseous product; and    -   viii) means for withdrawing a third oxygen-enriched stream,        which is also enriched with krypton and with xenon relative to        the second oxygen-enriched stream, from the auxiliary column,        characterized in that it includes means for sending, as reflux        stream to the auxiliary column, liquefied air or a liquid stream        enriched with nitrogen relative to a liquid air stream sent to        the medium-pressure column.

According to other optional aspects, the plant includes:

-   -   a purification column, means for sending the third        oxygen-enriched stream into the top of the purification column        and means for withdrawing a fourth oxygen-enriched stream,        constituting a mixture enriched with krypton and xenon, at least        a few theoretical stages lower down in the column; and    -   an exchange line in which the liquefied air and/or the liquid        enriched with nitrogen relative to the air is produced by heat        exchange with the oxygen-rich liquid stream coming from the        bottom of the low-pressure column, optionally after a        pressurization step.

The invention will now be described with reference to FIGS. 1 to 9,which are diagrams showing the principle of plants according to theinvention.

In the example shown in FIG. 1, a double air separation column comprisesa medium-pressure column K01 and a low-pressure column K02 that arethermally coupled by means of a principal vaporizer E02 that is used tocondense at least part of the gaseous overhead nitrogen of the columnK01 by heat exchange with oxygen from the bottom of the column K02.

An argon column K10 is fed with an argon-enriched fluid 7 coming fromthe low-pressure column K02 and an argon-enriched liquid 9 is returnedfrom the argon column K10 to the low-pressure column K02. An argon-richstream ARGON is withdrawn from the top of the column K10.

In the case of pumped units, a portion of the dry and decarbonized airis compressed in an air booster (not illustrated) up to the pressuresufficient to allow vaporization of the optionally pumped oxygen. It isthen condensed in the main exchange line (not illustrated). At the coldend of the main exchange line, this flow is expanded in a valve or in ahydraulic turbine. The liquid phase LIQ AIR of this fluid can then bedistributed as streams 1, 3 and 5 between the medium-pressure columnK01, the low-pressure column K02 and the auxiliary column K05,respectively. The liquid contains 78 mol % nitrogen.

The other portion of the medium-pressure air MP AIR is cooled in themain exchange line and sent to the bottom of the medium-pressure columnK01.

The principle of the present invention is to concentrate the krypton andthe xenon in a rich liquid RL2, which will then be treated in anauxiliary column K05.

Two rich liquids RL1 and RL2 are therefore withdrawn from themedium-pressure column K01, namely a “conventional” rich liquidwithdrawn from an intermediate level a few stages above the bottom ofthe column and containing a small quantity of krypton and of xenon, RL1,and a rich bottoms liquid concentrated with krypton and xenon, RL2. This“conventional” rich liquid RL1 can then be sent to the column K02 afterhaving been subcooled.

The rich bottoms liquid RL2 is sent to the K10 argon mixture condenserE10 after subcooling (not illustrated). Stages are installed above thisequipment in order to concentrate the krypton and the xenon at the argonmixture condenser. This assembly constitutes the column K05. A portionof the reflux from this column is provided by a portion 5 of the liquidair LIQ AIR not feeding the column K01, after this has been subcooled.The other portion of the reflux is provided by a portion 15 of the meanliquid 11 conventionally sent to the column K02 via the line 13 andcontaining at least 80 mol % nitrogen. A gas 16 is withdrawn from theintermediate level of the column K05 below the points of refluxinjection, and constitutes the vaporized rich liquid. It is thenrecycled in the column K02. The overhead gas WN2′ from the column K05constitutes a portion of the waste gas WN2 leaving the cold box.

The purge PURGE from the mixture condenser E10 contains most of thekrypton and xenon that are present in the air and have been treated bythe columns K01 and K05. This flow feeds a device for concentrating therare gases. For example, it may be sent into the weak krypton/xenonmixture column (K90). The bottom of this column contains the product tobe beneficiated. The vapor 17 coming from the column K90 is sent backinto the bottom of the column K05.

The column K90 is heated by a stream of air forming a fraction of the MPAIR. The liquefied air thus formed may be sent back to themedium-pressure column K01 and/or to the low-pressure column K02.

The production of liquid oxygen LO is withdrawn as bottoms from thecolumn K02, level with the principal vaporizer E02. Unlike theconventional scheme for krypton and xenon production, the principalvaporizer is therefore massively purged.

The liquid oxygen LO is preferably pressurized by a pump and thenvaporized in the exchange line or in a dedicated vaporizer, by heatexchange with the pressurized air. Alternatively, a nitrogen cycle mayserve to vaporize the liquid oxygen LO.

In the following figures, various alternative embodiments deriving fromFIG. 1 will be presented. The elements common with FIG. 1 will not bedescribed a second time.

In the case of FIG. 2, all of the liquid air LIQ AIR coming from themain exchange line is sent into the column K01. An intermediate fluid inliquid form 1′ is withdrawn from the column K01 (preferably at the levelwhere the liquid air is introduced or at a level above this level).Next, after having been subcooled, it is distributed between the columnK02 and the column K05 as two streams 3 and 5. Stream 11 containing atleast 80 mol % nitrogen is sent to the top of the column K05.

In the case of FIG. 3, based on FIG. 1, one top section of the columnK05 has been removed. The reflux from this column is provided only byliquid air 5, preferably subcooled. This liquid air is produced byvaporization of the liquid oxygen LO pumped and vaporized in theexchange line. All the lean liquid 13 is sent to the low-pressure columnK02.

In addition, all of the liquid air LIQ AIR present at the outlet of theexchange line can be withdrawn from the column K01 (preferably at thepoint of introduction of the liquid air) and then distributed betweenthe column K02 and the column K05 after having been subcooled, as shownin FIG. 4.

In the case of FIGS. 5 and 6, based on FIGS. 3 and 4, the waste gas WN2′from the column K05 is sent back into the column K02 below the point ofinjection of the lean liquid 13.

In the case of FIG. 7, based on FIG. 5, the stream 16 is omitted andreplaced with a stream of waste nitrogen WN2′ sent from the top of theauxiliary column K05 to an intermediate point on the low-pressurecolumn.

In all the figures described above (FIGS. 1 to 7), it is possible tocouple the plant with the conventional scheme for producing krypton andxenon. To do this, it is necessary to install stages for enrichment ofthe bottom in the column K02. The liquid oxygen LO is produced a fewstages above the principal vaporizer E02. A purge 21 is withdrawn levelwith the principal vaporizer E02. It contains about 70 mol % krypton andall of the xenon present in the column K02. It is sent to the column K90in order to recover the rare gases.

An example is given in FIG. 8.

In all the above figures (FIGS. 1 to 8), the coproduction of argon ismentioned. However, it is possible to fit the plants described above toa unit that does not produce argon. For example, it is sufficient toinstall an exchanger for condensing a fraction of the gas 7 withdrawnfrom the column K02. Once liquefied, it is sent (9) into the column K02.This thus provides the reboiling in the column K05.

An example is given in FIG. 9.

In the case of a scheme with a blowing turbine, the blown air is sentinto the bottom of the column K05 so as to recover the krypton and xenonthat it contains.

In addition, the schemes illustrated in FIGS. 1 to 9 may also includedistillation assemblies, such as for example an Etienne column (a columnthat operates at an intermediate pressure between the medium and lowpressures and fed with rich liquid). In this case, it is possible tomodify the top condenser of an Etienne column, by replacing the argoncolumn K10 of FIGS. 1 to 9 with an Etienne column according to the sameprinciple: addition of stages above the condenser in order toconcentrate the rare gases.

It is perhaps also advantageous not to send all of the liquid air intothe top of the auxiliary column but to introduce, at this inlet of thecolumn, only a stream that ensures an L/V (the ratio of the fallingliquid flow rate to the rising gas flow rate in the distillationsection) needed to concentrate the Kr and Xe in the bottom of K05,thereby limiting the oxygen concentration in the bottom of K05. Theremainder of the liquid air stream is then sent, with the rich liquidRL2, into the bottom of the auxiliary column.

1-12. (canceled)
 13. A method which may be used producing oxygen andrare gases by distillation, said method comprising: a) separating atleast one stream of cooled and purified air in a medium pressure column;b) withdrawing at least a first nitrogen enriched stream from saidmedium pressure column; c) sending at least part of said first nitrogenenriched stream to a low pressure column; d) withdrawing an intermediatestream from an intermediate level of said medium pressure column; e)withdrawing a first oxygen enriched stream from the bottom of saidmedium pressure column, wherein said first oxygen enriched stream isricher in oxygen than said intermediate stream; f) sending said firstoxygen enriched stream to the bottom of at least one auxiliary column;g) withdrawing a second nitrogen rich stream from the top of said lowpressure column; h) withdrawing a second oxygen rich liquid stream fromsaid low pressure column; i) withdrawing a third oxygen enriched streamfrom said auxiliary column, wherein said third oxygen enriched stream isalso enriched with xenon and krypton as compared to said second oxygenrich stream; j) sending said intermediate stream to said low pressurecolumn; and k) sending at least one liquid reflux stream to saidauxiliary column, wherein said reflux stream comprises about 78 mol %nitrogen.
 14. The method of claim 13, further comprising vaporizing saidsecond oxygen rich liquid stream to obtain a gaseous product.
 15. Themethod of claim 13, further comprising indirectly sending at least partof said first nitrogen enriched stream to said low pressure column. 16.The method of claim 13, further comprising: a) sending said third oxygenenriched stream to the top of a purification column; and b) withdrawinga final oxygen enriched stream from a final location on saidpurification column, wherein: 1) said final oxygen enriched streamcomprises a mixture enriched with krypton and xenon; and 2) said finallocation is at least about three theoretical stages down from the top ofsaid purification column.
 17. The method of claim 13, wherein saidliquid reflux stream comprises at least one member selected from thegroup consisting of: a) liquefied air; and b) a liquid stream richer innitrogen than a liquid air stream which is sent to said medium pressurecolumn.
 18. The method of claim 13, wherein the bottom of said auxiliarycolumn is heated by an overhead gas from an argon column.
 19. The methodof claim 17, further comprising producing said liquid reflux stream byheat exchange with said second oxygen rich liquid stream.
 20. The methodof claim 19, wherein said heat exchange takes place after apressurization step.
 21. The method of claim 17, wherein said refluxstream comprises at least about 80 mol % nitrogen.
 22. The method ofclaim 17, wherein said liquefied air does not originate from said mediumpressure column.
 23. The method of claim 13, wherein said reflux streamis richer in nitrogen than said intermediate stream.
 24. The method ofclaim 13, further comprising withdrawing at least about 10% of all theoxygen produced by said method from said low pressure column.
 25. Anapparatus which may be used for producing oxygen and rare gases bydistillation, said apparatus comprising: a) at least one medium pressurecolumn; b) a low pressure column; c) an auxiliary column; d) a firstdistribution means for sending at least one stream of cooled andpurified air to said medium pressure column, wherein said cooled andpurified stream is then separated; e) a first withdrawing means forwithdrawing at least a first nitrogen enriched stream from said mediumpressure column; f) a second distribution means for sending at least aportion of said first nitrogen enriched stream to said low pressurecolumn; g) a second withdrawing means for withdrawing a second nitrogenrich stream from the top of said low pressure column; h) a thirdwithdrawing means for withdrawing an intermediate stream from anintermediate level of said medium pressure column; i) a thirddistribution means for sending a first oxygen enriched stream from saidmedium pressure column to said auxiliary column, wherein said firstoxygen enriched stream is richer in oxygen than said intermediatestream; j) a fourth distribution means for sending a liquid refluxstream to said auxiliary column; k) a fourth withdrawing means forwithdrawing a second oxygen rich stream from the bottom of said lowpressure column, wherein said second oxygen rich stream is withdrawn asa liquid and is suitable for use as product; l) a fifth withdrawingmeans for withdrawing a third oxygen enriched stream from said auxiliarycolumn, wherein: 1) said third oxygen enriched stream is richer inkrypton and xenon than said second oxygen rich stream; and 2) said fifthwithdrawing means comprises a fifth distribution means for sending asecond reflux stream to said auxiliary column, wherein said secondreflux stream comprises at least one member selected from the groupconsisting of: i) liquefied air; and ii) a liquid stream richer innitrogen than a liquid air stream which is sent to said medium pressurecolumn.
 26. The apparatus of claim 25, wherein said second oxygen richliquid stream is vaporized prior to withdrawal in order to obtain agaseous product.
 27. The apparatus of claim 25, wherein said seconddistribution means indirectly sends at least a portion of said firstnitrogen enriched stream to said low pressure column.
 28. The apparatusof claim 25, further comprising: a) a purification column; b) apurification distribution means for sending said third oxygen enrichedstream into the top of said purification column; c) a purificationwithdraw means for withdrawing a final oxygen enriched stream,wherein: 1) said final oxygen enriched stream comprises a mixtureenriched with krypton and xenon; and 2) said final oxygen enrichedstream is withdrawn at least about three theoretical stages down fromthe top of said purification column.
 29. The apparatus of claim 25,further comprising an exchange line, wherein said second reflux streamis produced in said exchange line by heat exchange with fourth oxygenrich stream.
 30. The apparatus of claim 29, wherein said second refluxstream is produced after a pressurization step.